Here's a link to a must read from The Dirt - http://dirt.asla.org/2013/11/16/what-makes-soils-healthy/
Here are a few quotes to entice you to click over -
“Growing plants is the goal,” said James Urban, FASLA, Urban Trees + Soils, at the 2013 ASLA Annual Meeting in Boston. To grow healthy plants, one needs healthy soils, and landscape architects who understand soils and know how to call a soil scientist. In a wide-ranging talk, Urban and his co-presenter, soil scientist Norm Hummel, discussed the qualities and properties of healthy soils and how landscape architects can specify new dirt the right way, particularly in challenging damaged urban landscapes.
"Urban said there are eight critical properties of soils, which soil biologists can test to determine if soils meet specifications. They include structure, texture, density, nutrients, PH, organic matter, and density, which are all “inter-connected.”
More often than not, Urban said trees and plants don’t do well because of the physical properties of soils rather than the chemical. If something goes wrong — a tree is stressed, shows early fall color, or even dies — landscape architects may be planting the wrong trees and plants for the soil types.
Some details on soil’s physical properties: The structure of soils has to do with how well-glued together the soil particles are. Particles are attracted to other particles — and organic matter glues them together. Clay soil has a strong structure due to the stickiness of the soil. Silt soil has a weaker structure, while sand has no structure at all. Sandy soils are useful in areas that need to drain.
Urban added that man-made mixed soils are very different from natural soils. Mixed soils include soils that have been broken apart and put back together.
Soils are also made up of spaces or voids where water can flow. Ideal forest soils have a void space of about 50 percent, while urban compacted soils are around 20-30 percent. With the Sustainable Sites Initiative (SITES), Urban said more landscape architects will need to measure soil structure.
Soil texture is also important to examine. Clay, silt, and sand all have different surface areas given the unique sizes of the particles. Fine sand is .24mm, while silt is 2.4mm, and clay, nearly 24mm. Just within the family of sand, there are huge differences as well, with fine sand having properties distinct from coarse grains."
But compost is most often added to soils to boost the amount of organic matter. This is often used with disturbed urban soils that have suffered from erosion and compaction. Compost types include yard waste (grass, wood chips), bio-solids (treated municipal sewage), animal manure, and mixed waste. Some regional compost specialties include pine bark and rice hulls. Hummel added that soils have a “disease suppressive capacity.” Still, he cautioned against the practice of using 90 percent compost and 10 percent soil, saying that a “tree planted in that will simply fall over or die.”
Now, go over and read the rest!